1. Field of the Invention
This invention applies to saturated vapor passages where high velocity conditions can be advantageously slowed by means of a flow diffuser that simultaneously causes the static pressure to rise as vapor velocity is decreased by increasing the flow area. An ideal diffuser would reversibly convert the high initial kinetic energy to potential energy, thus increasing the static pressure.
2. Description of the Prior Art
Diffusers, for example, are commonly employed in steam turbines. Effective diffusers can improve turbine efficiency and output. Unfortunately, the complicated flow patterns existing in such turbines as well as the design problems caused by space limiations make fully effective diffusers almost impossible to design. A frequent result is flow separation that fully or partially destroys the ability of the diffuser to raise the static pressure as the steam velocity is reduced by increasing the flow area. This is often caused by a vapor boundary layer that gets thicker along the diffuser surface in the direction of flow ultimately permitting the flow separation mentioned above.
In operation, the turbine shaft and last stage rotating blades rotate at high speed, often at 3600 rpm, with over 1800 feet per second top speed. Steam exhausts from the last stage buckets or rotating blades with axial velocity approaching sonic velocity and, in addition, a variable amount of residual whirl. Up to a limit, the lower the absolute static pressure at the discharge of the last stage rotating blade, the greater is the turbine available energy and the turbine output. The limit occurs when the axial steam velocity in the annular space immediately downstream from the last stage rotating blade equals sonic velocity. This is typically about 1220 feet per second for wet steam at the discharge of the low pressure turbine. Any further dropping of static pressure below this condition will not result in increased output and may in fact, slightly reduce output.
For most turbines, during most operating conditions, the exhaust static pressure is above the limit described above. As a result, a system that lowers the static pressure at the last stage exhaust will improve cycle efficiency and turbine output. This is the purpose of the diffusers that currently exist in most turbine section exhausts.
In the last stage example mentioned above, the condenser hotwell pressure is essentially established by the condenser tube geometry, the temperature of the circulating water, and the heat to be removed from the steam exhausted from the turbine.
The static pressure of the steam exiting the exhaust hood and entering the condenser is usually close to the pressure existing in the hotwell, depending on local flow interferences such as pipes and side wall obstructions and feed water heaters. It should be recognized that if there are significant interferences, the pressure at the discharge of the exhaust hood will be higher than the hotwell.
The static pressure at the discharge side of the diffuser will be higher than that of the exhaust hood discharge by the amount of pressure drop required to turn the flow from nearly axial to vertical and by the necessary pressure drop caused by passage of pipes, struts, and other such interferences.
It should be also noted that for downward exhaust hoods the loss from the diffuser discharge to the exhaust hood discharge varies from top to bottom. At the top, much of the flow must be turned 180.degree. to place it over the diffuser and inner casing, then turned downward. Pressure at the top is thus higher than at the sides which are in turn higher than at the bottom.
The static pressure at the annulus immediately downstream of the last stage rotating blade will be lower than that at the discharge of the diffuser by the amount of successful diffusion, that is, the degree to which the reduced average velocity has been successfully turned into higher static pressure as the steam flows along the diffusing path.
This will be harmfully affected by the strong tendency of the high velocity flow to separate off either the diffuser at the outer periphery or the inner flow surface usually called the bearing cone.
In the most successful of existing downward exhaust hoods, the average static pressure at the discharge of the last stage is close to the static pressure at the hotwell. Most turbines are poorer than this. Reduction of diffuser and bearing cone flow separation would provide significant performance improvement.
There is a need for improved diffusers in both existing and new steam turbines. It is believed that many other fluid flow diffusers where the fluid is saturated vapor could also benefit from the present invention